RSI: how to use and code the Relative Strength Index.

The RSI is probably the first indicator every trader learns — and also one of the most misused. Most memorize "70 sell, 30 buy" and stop there, which leads to losing money in strong trends. This guide goes further: explains what the RSI actually measures, where it shines, where it fails, and delivers the code for you to plug into a bot.

01What the RSI actually measures

RSI stands for Relative Strength Index, created by J. Welles Wilder in 1978. It's a momentum oscillator: ranges from 0 to 100 and measures the speed and magnitude of recent price changes. In simple terms: it compares the size of recent gains against the size of recent losses.

• RSI above 70: overbought — price rose fast, may be "stretched".
• RSI below 30: oversold — price fell fast, may be "stretched" downward.
• RSI at 50: balance between buying and selling force.

02The formula (to understand, not memorize)

You don't need to calculate by hand — every platform does —, but understanding the formula prevents misuse:

RSI = 100 − [ 100 / (1 + RS) ]

where RS = average gains / average losses (over the period)

The default period is 14. The practical consequence of the formula: when there are only gains in the period, RS tends to infinity and RSI goes to 100; when there are only losses, RSI goes to 0. That's why it "saturates" — and that's exactly where the trap we'll see ahead lies.

03The 3 real uses (beyond the obvious)

1. Overbought / oversold

The classic use. But attention: in ranging markets it works well; in strong trends, the RSI stays "stuck" at the extreme (above 70 for days in a strong rally) and the "sell" signal makes you trade against the trend — expensive mistake.

2. Divergences (the most valuable use)

Here's the RSI gold. Divergence happens when price and RSI disagree:

• Bearish divergence: price makes a higher high, but RSI makes a lower high → the upward movement is losing strength, possible downward reversal.
• Bullish divergence: price makes a lower low, but RSI makes a higher low → the decline is weakening, possible upward reversal.

3. The 50 line as trend filter

Underrated use: RSI consistently above 50 confirms uptrend; below 50, downtrend. Great as a filter for other strategies — only buy with RSI above 50, for example.

04Coding the RSI

Here's the ready-to-use code for the two most popular platforms. Note that both Pine and Python have RSI in their libraries — you rarely calculate by hand, but I show the manual calculation in Python so you understand the internals.

//@version=5
indicator("RSI com Zonas")

periodo = input.int(14, "Period")
rsi = ta.rsi(close, periodo)

plot(rsi, "RSI", color=color.blue)
hline(70, "Overbought", color=color.red)
hline(30, "Oversold", color=color.green)
hline(50, "Middle", color=color.gray)

// crossunder alert
if ta.crossunder(rsi, 30)
    alert("RSI entered oversold")

import pandas as pd

def calcular_rsi(precos: pd.Series, periodo=14) -> pd.Series:
    delta = prices.diff()
    gain = delta.clip(lower=0)
    loss = -delta.clip(upper=0)
    # Wilder's exponential moving average
    avg_gain = gain.ewm(alpha=1/periodo, adjust=False).mean()
    avg_loss = loss.ewm(alpha=1/periodo, adjust=False).mean()
    rs = avg_gain / avg_loss
    return 100 - (100 / (1 + rs))

# usage
df["rsi"] = calcular_rsi(df["close"])
if df["rsi"].iloc[-1] < 30:
    print("Oversold — evaluate buy with confirmation")

05Which period to use

The default 14 is a good balance, but you can adjust based on style:

• Short periods (7-9): more sensitive RSI, more signals, more noise. For scalping and fast day trading.
• Period 14 (default): balance. Works in most cases.
• Long periods (21+): smoother RSI, fewer signals, more reliable. For swing trading.

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